WD20EARS - Noise when unplugged

WD Internal Drives Desktop & Mobile Drives
1

Hi
My new WD20EARS sometimes makes a noise. It’s almost identical to read/write sounds, but not as loud. Activity LED does not light when that noise occurs. It even happens when I unplug the drive (it’s in external case), so it can’t be caused by system accessing the disk. This noise does not occur requllary, frequency seems to be random, and it’s quite short (0.5-2sec).

Should I worry?